Give reason for the following:
a. Why potassium is reactive than rubidium ?
b. Irrespective of the alkali metal dissolved in liquid ammonia, dil solution is always blue coloured.

Explanation

a. The atomicsize of potassium is smaller as compared to rubidium, thus the valence shell electron `(ns^(1))` is more loosely bound in rubidium. As a result, ionisation ethalpy of potassium is more than rubidium and hence potassium is less reactive than rubidium.
b. Alkali metals dissolve in liquid ammonia to give ammoniated metal ion and ammoniated electron.
`M_((s))+(x+y)NH_(3)rarr underset("metal ion")underset("Ammoniated")([M(NH_(3))_(x)])^(o+) + underset("electron")underset("Ammoniated")([e(NH_(3))_(y)])^(Θ)`
The blue colour is because of ammoniated electron which absorbs energy corresponding to the red region of the visible light and gives complimentary blue colour.